Hi Hans,
GIFs were OK years ago, before PNG. They have support for only 256 colours in their index tables, though, so photos often look grainy. JPGs are good for most photos or other images with lots of shading and colour gradients. PNGs are probably the best for schematics and diagrams. For schematics, GIFs would have sharper images than JPGs.
There are some free image editing software packages around. I mostly use the GIMP (
GNU Image Manipulation Program). There is a version for Windows
**broken link removed**. It might be overkill for just cropping screenshots, though. It can read and write almost any image file format you can think of.
Torben